Cerium oxide is a high-functional ceramic material widely used for catalysts, phosphors, cosmetics, abrasives, etc. With the advent of the STI process in the semiconductor device field, it is drawing attention as inorganic abrasive, the key material of the slurry used in the next-generation chemical mechanical polishing (hereunder referred to as “CMP”) process.
Properties required for the CMP slurry used in the STI CMP process are removal rate of the silicon oxide film, selectivity between the silicon oxide film and the silicon nitride film, low scratch production of the polished surface, etc. To obtain such a slurry, the cerium oxide powder, or the abrasive, should have ideal particle size, shape, oxidation property, crystallinity, strength, etc.
High temperature solid state method is a method for synthesizing new material through decomposition of volatile substance such as carbonate or thermal reaction of different materials by heating near the melting point to increase atomic activity.
Powder synthesis by high temperature solid state method is limited in that particle growth is difficult to control, depending on composition, particle size, coagulation status, impurity composition and content, etc. For example, materials including carbonate are decomposed in the temperature range of 700-1000° C. In the process of decomposition and emission of carbon dioxide, distance between particles increases and particle shape is not controlled. As a result, particle growth or shape control is limited.
In high temperature treatment of ceramic materials, a material having a lower melting temperature is often used as flux in order to reduce sintering temperature and obtain fine particles. The flux offers a liquid phase at a temperature lower than the melting temperature of the main material, thereby improving inter-particle contact and facilitating melting of the main material sintering, which makes material transfer easier.
There were reports on synthesis of cerium oxide powder by high temperature treatment of cerium salts (cerium carbonate, cerium chloride, cerium sulfate, cerium hydroxide, etc.). However, in most cases, high temperature treatment was performed for the simple purpose of phase transition to cerium oxide. Thus, control of particle shape or size was limited.
According to the conventional synthesis method of cerium oxide proposed by Matijevic et al., raw materials such as Ce(SO4)24H2O, (NH4)4Ce (SO4)42H2O, (NH4)2Ce(NO3)6, and so forth are sealed in a Pyrex tube and heated to a pre-determined temperature to precipitate cerium hydroxide. Then, it is calcined at about 600° C. to obtain particles in the form of hexagonal plate or sphere (Wan Peter Hsu, Lena Roannquist, Egon Matijevic, “Preparation and Properties of Monodispersed Colloidal Particles of Lanthanide Compounds. 2. Cerium(IV),” Langmuir, 4, 31-37 (1988)).
With the technique reported in this paper, control of particle shape and size is possible at very low concentration. However, at high concentration, because mixture crystal of needle-shaped cerium hydroxide and cerium sulfate is obtained, shape control is difficult. Besides, productivity is also low. Moreover, the resulting powders are present as hydrates of a size of several nanometers, which makes shape maintenance during high temperature treatment very difficult.
Takuya Tsuzuki et al. synthesized nano-sized uniform cerium oxide particles using cerium chloride (CeCl3) and sodium hydroxide (NaOH) through mechanochemical and calcination processes. They reported that cerium hydroxide can be synthesized by mechanochemical reaction by crushing cerium chloride adding sodium hydroxide (NaOH) and sodium chloride (NaCl) with a steel ball, and that spherical, nano-sized cerium oxide particles are obtained by calcining the cerium hydroxide at a temperature of 500° C. or higher (Takuya Tsuzuki, Paul G. McCormick, “Synthesis of Ultrafine Ceria Powders by Mechanochemical Processing,” J. Am. Ceram. Soc., 84(7), 1453-58, (2001)).
Since this technique has to use excess sodium chloride (NaCl) as spacer in the calcination process to control particle coagulation, washing off of the sodium chloride is difficult. Also, coagulation control is impossible at a temperature higher than the sublimation temperature of sodium chloride. And, only cerium chloride (CeCl3) can be used as starting material.
According to the synthesis method of cerium oxide used as abrasive material for CMP slurry, which was invented by Hitachi Chemical of Japan, (Korean Patent Application No. 10-2001-7014923), cerium salts (cerium carbonate, cerium sulfate, cerium oxalate) are baked in a rotary kiln at 600-1000° C. to obtain cerium oxide powder, which is crushed using a jet mill or a beads mill to prepare a cerium oxide abrasive.
In this technique, very tiny sintered particles (several nanometers) with weak strength are synthesized to reduced scratch during the CMP process. Consequently, particle shape is quite non-uniform and particle size is also impossible during the thermal treatment process.